I'm almost 100% sure that Kagawa initially was signed to "replace" Rooney in the no.10 position. The sudden signing of RVP messed these plans up, though. Even Fergie was surprised by how these things turned out, as he never expected Arsenal to sell us RVP.
In my opinion, there are 3 types of "attacking midfielders" at top level.
1. The Versatile Striker
This is where players like Rooney, Suarez and Tevez fit in. These are basically strikers who are "too skilled" to only score goals, yet too much of a goal threat to be pushed out wide or back into CM. As a result, they end up playing in then hole behind the striker, as this is the place where they are most useful.
2. The Offensive Playmaker
This is where players like Kagawa, Iniesta, Silva and Mata fit in. These are basically midfielders who have too much attacking fibers in their bodies to be played in CM. Their first-touch, close control, vision, and link-up abilities are the reasons for why they are so useful in the AM position, and also what seperates them from other players.
3. The Versatile Winger
These players usually play out wide, but like to cut in. They are basically tailored for the modern 4-2-3-1 formation. Nevertheless, they are still more wingers than they are AM's, so that is why they mostly only play in the hole behind the striker as a temporary solution. Reus, Götze and Hazard are seems to fit this description.
In my personal opion, it's the Offensive Playmakers that make the best no.10's. It's simply because that is their specialty. Players like Rooney don't really have a specialty. That is their beauty. They can play well in pretty much any position. Rooney is therefore a much better striker and CM than Kagawa. As an AM though, I think Kagawa is better(depending on the style of the team, of course).
Kagawa is therefore pushed out left for the following reasons:
1. We got to sign RVP out of the blue(i.e. change of plans).
2. He's not used to the PL yet(and he's also been injured).
3. In our current style of play, we are slightly better off with Kagawa out left and Rooney as our no.10.
Once Kagawa finds his Dortmund form, things might get interesting to say the least. We'll be stuck with three choices then. 1) Let Kagawa take Rooney's role, 2) Change our style of play to accommodate Kagawa, or 3) Bench Kagawa. I just can't see number 3 happening. He's too good for that. It all comes down to number 1 or 2.